She's a favorite princess on the drama by the reigning king Tae Jo, and is played by actress Kang Han Na, who rose to fame for her appearance in film 'Empire of Lust', where she also worked with Kang Ha Neul.
One reason netizens and viewers of the drama have been raving is that she's been pointed out as Sulli's look-alike. Another is just that she's drop dead gorgeous!
Netizens spared no compliments for Kang Han Na on 'Scarlet Heart: Ryeo', commenting, "Kang Han Na is pretty as hell TT I get so mad at her in the drama but I can't say anything because she's pretty..", "She really looks like Sulli in the first GIF kekeke", "Her eyes are similar [to Sulli's]", "She's the prettiest princess", etc.
Take a look at a couple of screen captures of her from the drama below, plus a few selcas. Do you think she looks like Sulli?